Process for producing polyester film

ABSTRACT

A molten polyester sheet is solidified by cooling it on the surface of a mobile cooling medium on which a water film has been formed. The sheet is stretched at least 4.5-fold in overall stretching ratio in a longitudinal direction through a plurality of stretching steps. Since casting is conducted via the water film, water is absorbed in the surface layer and the interior of the cast sheet, thus facilitating orientation by stretching in the longitudinal direction and lowering the stretching temperature. As a result, stretching with a high stretching ratio can be attained by multi-stage stretching without conducting high-temperature stretching. Thus, this process can accelerate film formation, improve productivity, and provide a film having reduced unevenness in thickness, less surface roughness and excellent abrasion resistance.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process for producing a polyesterfilm, more specifically to a process for producing a polyester filmwherein a molten polyester sheet is solidified by cooling and the sheetis stretched in its longitudinal direction.

BACKGROUND ART OF THE INVENTION

A polyester film is produced, generally, by casting a molten polyestersheet, delivered from a die onto the surface of a mobile cooling mediumthat is continuously moving so as to solidify the molten polyester sheetby cooling, heating the solidified sheet and stretching the sheet in itslongitudinal direction. The film stretched in the longitudinal directionis a uniaxially stretched film. If the film is further stretched in itstransverse direction, after stretching it in the longitudinal direction,a biaxially stretched film can be obtained.

As high-speed film forming techniques in the production of polyesterfilm, a method for increasing the casting speed by casting a moltenpolyester sheet onto the surface of a mobile cooling medium having aliquid layer, such as a water layer on the surface, and a method forincreasing a longitudinal draw ratio by stretching a sheet in itslongitudinal direction through a plurality of stretching steps not lessthan two steps, are individually known, respectively.

The former method is disclosed in, for example, U.K. Patent 1,140,175and JP-B-SHO-58-35133.

However, there exists a limit to the increase in the film forming speedafter the longitudinal stretching of a sheet only by cooling andsolidifying a molten polyester sheet on the surface of a mobile coolingmedium having a liquid layer on the surface. Moreover, in this castingprocess, it is important to form a uniform liquid layer on the surfaceof a mobile cooling medium and interpose the formed liquid layeruniformly between the surface of the mobile cooling medium and themolten polyester sheet. If the liquid layer is not uniformly formed orinterposed, it causes the cooled and solidified sheet to have surfacedefects.

The latter method, i.e. the method for stretching a polyester sheet inits longitudinal direction with a high stretching ratio through aplurality of stretching steps, is disclosed in, for example,JP-B-SHO-52-10909 and JP-B-SHO-52-33666. These publications teach aprocess for stretching a polyester sheet at a total draw ratio of notless than 4.5 times the original length by multi-stage stretchingcomprising a high-temperature stretching process and a low-temperaturestretching process. Since this multi-stage stretching can achieve a highlongitudinal draw ratio, such as a ratio of about two times of2.5-3.5-fold which is disclosed in JP-B-SHO-38-23489 etc., or a ratiohigher than that value such as 5-9 times, high-speed film formationprocess can be attained, including speeds at greater than 200 m/min.

In this multi-stage stretching, however, it is necessary to increase thestretching temperature in comparison with a usual stretching temperatureof 80°-95° C. in order to ensure a high longitudinal draw ratio.Therefore, in this stretching process, problems are liable to occur suchas (1) the thickness variation of the film obtained deteriorates becausethe draw ratio is likely to disperse, (2) the surface of the filmobtained becomes rough because the film is likely to adhere to ahigh-temperature stretching roll, and (3) the abrasion resistance of thesurface of the film finally obtained decreases because the degree ofcrystallinity of the film increases and the surface layer of the film islikely to be abraded. Moreover, because the polyester sheet (or film) isstretched at a high temperature, an oligomer precipitates from thesheet. The precipitated oligomer soils the surface of a stretching rollin a short period of time. If the surface of the stretching roll issoiled, surface defects occur on the obtained film, and the productivityof the film is greatly decreased as the soiled roll must be cleaned orexchanged. Furthermore, in this conventional multi-stage stretching, ifthe film is stretched at a total draw ratio of not less than 4.5 timesthe original length at a temperature of 80°-95° C. which is a usualstretching temperature, the film formation is not stable and qualitydefects are caused such as a rough surface and degraded transparency.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producinga polyester film which can form a high-quality polyester film at a highspeed with a stable productivity.

Another object of the present invention is to provide a process forproducing a polyester film which can suppress the thickness variationand the surface roughness of an obtained film to small extents, canobtain a film surface having a high abrasion resistance, and can achievean excellent productivity without soiling stretching rolls, when thefilm is stretched at a total draw ratio of at least 4.5 times theoriginal length in its longitudinal direction through a plurality ofstretching steps in order to achieve the above high speed filmformation.

To accomplish the above objects, a process for producing a polyesterfilm according to the present invention comprises a process wherein,after a molten polyester sheet is solidified by cooling it on thesurface of a mobile cooling medium on which a water layer is formed, thesheet is stretched at a total draw ratio of at least 4.5 times theoriginal length in its longitudinal direction through a plurality ofstretching steps.

In the present invention, "polyester" is a generic term of a polymerwhich can be obtained by the condensation polymerization of glycol anddicarboxylic acid and which has an ester bond in its principal chain.Typical glycols are ethylene glycol, butanediolhexylene glycol,cyclohexanedimethanol and neopentyl glycol etc., and typicaldicarboxylic acids are terephthalic acid, isophthalic acid, phthalicacid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid,cyclohexanedicarboxylic acid, adipic acid, sebacic acid,dodecanedicarboxylic acid, dimer acid and eicosanic acid etc.

Typical polyesters are polyethylene terephthalate, polybutyleneterephthalate, polyethylene naphthalate and polycyclohexylenedimethyleneterephthalate etc. Particularly polyethylene terephthalate ispreferable.

To such a polyester, a generally used additive, for example, stabilizer,viscosity conditioner, oxidation inhibitor, filler, slipping agent,antistatic agent, anti-blocking agent and mold releasing agent etc., maybe added by the volume which does not substantially decrease the effectaccording to the present invention.

In the process according to the present invention, a molten polyestersheet is solidified by cooling on the surface of a mobile cooling mediumon which a water layer is formed. Although a cylindrical cooling drum isusually used as the mobile cooling medium, the mobile cooling medium maybe constructed from a circulated belt having a plain surface. Namely, amobile cooling medium has a cooling surface which can move continuously.After the molten polyester sheet is cast onto the surface of the mobilecooling medium, the molten polyester sheet is cooled and solidifiedwhile moving together with the surface of the cooling medium. Thesolidified sheet is then continuously sent to the next process.

The water layer is formed by supplying water to the surface of themobile cooling medium. The molten polyester sheet is cast onto thesurface of the mobile cooling medium on which the water layer is formed.The sheet is cooled and solidified while moving together with thesurface of the cooling medium in such a state wherein the water layer isinterposed between the sheet and the surface of the cooling medium.

In this cooling and solidifying step of the molten polyester sheet,preferably an electrostatic charge is applied to the molten sheet. Forexample, JP-B-SHO-37-6142 or JP-B-SHO-48-29311 discloses anelectrostatic casting method. The contact state between the molten sheetand the cooling medium can be improved by continuously applying anelectrostatic charge to the molten sheet using such a casting method.This conventional electrostatic casting method sometimes cannot be useddepending upon the kind of polymerization catalyst or additives in thepolyester. In the present invention, however, since a water layer isinterposed between the sheet and the surface of the cooling medium, theelectrostatic casting method can be employed even with these types ofpolyesters. By use of the electrostatic casting method, the intimatecontact between the molten polyester sheet and the surface of thecooling medium can be obtained. Moreover a uniform meniscus portion(described later) can be formed because the contact line which extendsin the transverse direction of the sheet at a position where the moltenpolyester sheet comes into contact with the surface of the coolingmedium, can be defined as a substantially straight line.

As the method for forming a water layer on the surface of the mobilecooling medium, there is a method wherein air containing water vapor(moisture) is blown onto the surface of the cooling medium, which iscontrolled at a temperature lower than the dew point of the water vapor,and the water vapor is bedewed on the surface of the cooling medium(bedewing method); a method wherein water is applied to the surface ofthe cooling medium by a seepage roller or a transfer roller; and amethod wherein water vapor charged with static electricity is sprayedonto the surface of the cooling medium. The bedewing method is desirablein the present invention because the water layer should be formedpreferably thinner in the present invention. It is not necessary thatthe water layer be in the form of a continuous layer when it is appliedto the surface of the cooling medium. That is, the water layer may beapplied in the form of discontinuous water drops such as dew drops.

In the process according to the present invention, a required waterlayer is formed on the surface of the cooling medium at of the positionahead a position where the molten polyester sheet comes into contactwith the surface of the cooling medium. The water layer formed on thesurface of the cooling medium continuously reaches the position wherethe molten polyester sheet comes into contact with the surface of thecooling medium, associated with the movement of the surface of thecooling medium. In this position, the amount of the water continuouslyconveyed into the position by the moving surface of the cooling mediumand the amount of the water continuously conveyed out from the positionand sandwiched between the sheet and the surface of the cooling mediumare balanced, and therefore, a particular state is presented as if aconstant amount of water is retained at the position. In thisquasi-retention water area, a meniscus phenomenon (phenomenon that thefree surface of a liquid curves by its surface tension) happens betweenthe surface of the molten polyester sheet immediately before coming intocontact with the surface of the cooling medium and the surface of thecooling medium. At the same time, the quasi-retention water extendscontinuously in the transverse direction of the sheet substantially overthe entire width of the sheet, along the line at which the moltenpolyester sheet comes into contact with the surface of the coolingmedium. In the present invention, this quasi-retention water portionwhere the meniscus phenomenon happens is called the "meniscus portion".Even though the water layer is formed as discontinuous water drops onthe surface of the mobile cooling medium before reaching the meniscusportion, the state of water drops is dissolved when the water dropsreach the meniscus portion because the water drops are absorbed in themeniscus portion. Therefore, the water layer formed is continuouslyconveyed out from the meniscus portion as a uniform and continuous waterlayer interposed between the molten polyester sheet and the surface ofthe cooling medium. However, the water layer flowing into the meniscusportion must be formed so as to prevent (1) interruption of the meniscusportion, (2) fluctuation of the size of the meniscus portion and (3) thewater layer, to be flown into the meniscus portion, from directly cominginto contact with the molten polyester sheet. The entrapment of air fromthe meniscus portion into the portion between the molten polyester sheetand the surface of the cooling medium can be prevented by forming astable meniscus portion without fluctuation.

The relationship between the height "h" of the meniscus portion from thesurface of the cooling medium and the average thickness "d" of the waterlayer formed on the surface of the cooling medium at a position ahead ofthe meniscus portion is preferably such that h>d. If the relationship ish≦d, the fluctuation of the thickness of the water layer is transferredto the surface of the molten polyester sheet as it is, the surface ofthe sheet is liable to become an irregular surface like an orange peeland it becomes difficult to maintain a stable casting for a long time.

This height h of the meniscus portion can be easily controlled inaccordance with the surface tension of the polymer of the moltenpolyester sheet, the surface roughness of the molten sheet, thethickness of the molten sheet, the surface roughness and surface tensionof the cooling drum, adhesion means such a electrostatic casting, thedistance between the die delivering the molten sheet and the surface ofthe cooling medium, and the angle of the molten sheet at the positionwhere the sheet comes into contact with the surface of the coolingmedium etc.

The method for forming the water layer on the surface of the coolingmedium is preferably a bedewing method from the viewpoint that a uniformand thin water layer is required in the present invention. It isdifficult to form a uniform water layer by other methods. Since thewater layer can be sufficiently cooled by the cooling medium before itreaches the meniscus portion as long as the water layer is formed as auniform and thin layer, the boiling of the water can be prevented whenthe water layer reaches the position of the molten polyester sheet. Ifthe boiling of the water occurs, the surface of the polyester sheetbecomes irregular and this is not desirable.

The maximum diameter of dew drops formed on the surface of the coolingmedium by the bedewing method is not greater than 70 μm, preferably notgreater than 50 μm. When the diameter is greater than 70 μm,irregularity defect occurs on the surface of the sheet.

The number of the water drops is preferably not less than 50/0.1 mm²,more preferably not less than 70/0.1 mm². When the number is less than50/0.1 mm², entrapment of air is liable to occur, due to the water, thatin turn cause surface defects of the sheet.

The surface roughness of the cooling medium according to the presentinvention is preferably not greater than 0.04 μm in center line averageheight Ra. If the surface roughness is greater than 0.04 μm, the surfaceof the sheet becomes too rough, it becomes difficult to conduct thewater elimination described later, and the surface defects of the sheetdue to water drops are likely to occur. This is because the diameter ofthe water drops becomes difficult to be suppressed to not greater than70 μm. The maximum surface roughness Rt is preferably not greater than0.4 μm.

To form such a specified water layer precisely, it is necessary that arequired amount of water is newly supplied by a water supply means toform the required water layer, after the residual water remaining on thesurface of the cooling medium is eliminated by a water eliminating meansat a position after the sheet leaves the surface of the cooling medium.Since the residual water is left on the surface of the cooling medium asrelatively large scattered island-like water, it is difficult to formthe required water layer if such island-like water remains. As methodsfor eliminating the residual water, there is a method for eliminatingthe water by an air suction roll on which a nonwoven fabric havingmoisture absorption properties is provided (described later), a methodfor blowing off the water by an air knife, a method of combining bothmethods, and a method for blowing air by a non-contact air knife andsucking water by another air knife. However, the water eliminationmethod is not particularly restricted.

The method for forming a water layer after eliminating the residualwater is preferably a bedewing method as aforementioned. The diameter ofwater drops included in the air containing water vapor or moisture andto be supplied to the surface of the cooling medium must be alsosuppressed to a small diameter in order to suppress the diameter of dewdrops on the cooling medium. The water vapor having such a small waterdrop diameter may be obtained, for example, by introducing air into thewater contained in a tank, bubbling the water by the introduced air, andintroducing the air including water vapor generated by the bubbling tothe surface of the cooling medium. Of course, other methods areavailable.

In the process according to the present invention, side portions of thewater layer are formed preferably thicker than the central portion inthe transverse direction of the sheet when the water layer is formed onthe surface of the cooling medium. Although the condition depends on thekind and thickness of the sheet, if the thickness of the water layer inits side portions is equal to or smaller than that in its centralportion, the flatness of the film after casting deteriorates and thethickness variation of the film deteriorates, thereby decreasing thequality and the productivity.

The side portion of the water layer in which the water layer isthickened preferably extends not more than 200 mm from each sheet edgein the transverse direction of the sheet, more preferably not more than150 mm from the sheet edge. If the area is greater than 200 mm from thesheet edge, the flatness of the sheet deteriorates.

Moreover, the water layer is preferably formed also in the area outsideof the sheet edge in the transverse direction of the sheet in which thesheet does not exist, with a width of not less than 5 mm and a thicknesssimilar to the thickness of the inside portion. By such a formation ofthe water layer, the water near the sheet end portion can be preventedfrom completely evaporating by heat transfer from the sheet end portion.The flatness of the sheet can thus be better maintained and the curlingup of the sheet end portion can be prevented.

The method for forming the water layer having the larger thickness inthe sheet end area may be a method for varying the amount of watersupplied by a single water supply means in the transverse direction ofthe sheet or a method for providing an additional water supply means andsupplying the water for the sheet end area from the additional watersupply means.

In the process according to the present invention, the anti-coolingmedium-side surface of the sheet is preferably cooled at its endportions in the transverse direction of the sheet. Although thecondition depends on the kind and thickness of the sheet, if thiscooling is not conducted, the sheet end portion is likely to curl up,the flatness of the sheet end portion is likely to deteriorate and evencracks of the sheet end portion are likely to occur. Therefore, stablefilm formation sometimes cannot be achieved. When the cooling isconducted, however, the crystallization of the anti-cooling medium-sidesurface of the sheet end portions is prevented by the cooling and thesheet end portions are rapidly solidified. Accordingly, the curling upetc. of the sheet end portions can be prevented.

As a method for this cooling, either a method for disposing nozzlesdensely in the sheet delivering direction and continuously cooling thesheet end portions by discharging cooling air or water out from thenozzles along the sheet end portions or a method for bringing a nonwovenfabric etc. into contact with the sheet end portions and cooling thesheet end portions by cooling water supplied through the fabric, can beapplied. However, other methods may be applied. The cooling by water ismore preferable than the cooling by air from the viewpoint of coolingefficiency. A method for cooling the sheet end portions by continuouswater layer until the temperature of the sheet reaches a temperature ofnot higher than the glass transition temperature Tg+20° C.) ispreferable from the viewpoints of preventing the flatness of the sheetend portions from deteriorating and preventing the sheet end portionsfrom cracking.

Further, the area of this cooling is preferably an area not less than 10mm from the sheet edges in the transverse direction of the sheet and cancool the sheet edge surfaces perpendicular to the surface of the coolingmedium and the portions of the surface of the cooling medium outside ofthe sheet edges up to a position far of not less than 5 mm from thesheet edges in the transverse direction of the sheet. Such a coolingarrangement is advantageous in preventing the flatness of the sheet endportions from deteriorating and preventing the sheet end portions fromcracking.

In the process according to the present invention, the polyester sheetsolidified by cooling as described hereinabove is stretched at a totaldraw ratio of at least 4.5 times the original length in its longitudinaldirection through a plurality of stretching steps.

In the above casting according to the present invention, while themolten polyester sheet is solidified by cooling, the water on thesurface of the cooling medium is absorbed in the surface layer and theinterior of the polyester sheet itself, and thereafter, the sheet issent to a longitudinal stretching process. In the longitudinalstretching process, the orientation of the sheet due to the stretchingcan be easily performed because the sheet contains water. Therefore, thestretching temperature of the process can be reduced. As a result,multi-stage stretching with a high draw ratio can be possible withoutconducting high-temperature stretching, and film having a smallthickness variation, less surface roughness and excellent abrasionresistance can be obtained.

The method for multi-stage longitudinal stretching according to thepresent invention is not particularly restricted. As typical methods,there are a method wherein each stretching step is conducted between anupstream drive stretching roll and a downstream drive stretching rollwhich are disposed adjacent to each other in the sheet runningdirection, at least one stretching step at a temperature of not higherthan the yield point is conducted at an upstream side position, andthereafter, the final stretching step is conducted as a usual stretchingat a temperature higher than the yield point, and a method wherein thestretching with a low stretching speed is conducted at a draw ratio ofabout 2 times, and thereafter, the final stretching step is conducted soas to reach a target draw ratio. The latter method will be explainedmore concretely. The method is, for example, a method wherein the finalstretching step is conducted between an upstream drive stretching rolland a downstream drive stretching roll which are disposed adjacent toeach other in the sheet running direction, and stretching steps upstreamof the final stretching step in the sheet running direction areconducted between an upstream drive stretching roll and a downstreamdrive stretching roll, which are disposed in the sheet runningdirection, via free stretching rolls which are disposed between thedrive stretching rolls. In this method, the stretching speed in theupstream side stage is preferably as low as possible, for example, notgreater than 10,000%/min. In any method described in the above, thefinal total draw ratio is set to a ratio of not less than 4.5 times theoriginal length.

In the multi-stage stretching according to the present invention, thedraw ratio of each stretching step is preferably at least 1.1 times,more preferably at least 1.3 times, further more preferably at least 1.5times and still further more preferably at least 2.0 times. If the drawratio is less than 1.1 times, the thickness variation of the filmdeteriorates, the roughness of the film surface increases and theabrasion resistance of the film surface decreases. Moreover in thismulti-stage stretching, it is preferable that the draw ratio of thefirst stretching step is the lowest ratio among the plurality of drawratios and other draw ratios become higher in order in successivestretching steps, from the viewpoint of preventing the thicknessvariation from deteriorating. If the draw ratio of a stretching step ishigher than that of the next stretching step, the thickness variationdeteriorates. If the thickness variation becomes too bad, film breakageoccurs and stable film formation cannot be attained.

In the process for producing a polyester film described hereinaboveaccording to the present invention, the producing speed of the cooledand solidified sheet can be increased in the casting process as well asthe sheet with a high quality can be obtained by the casting method forinterposing a required water layer, and in the longitudinal stretchingprocess, the stretching temperature can be lowered because the sheetabsorbing water is stretched, and stretching with a high draw ratio canbe attained by the multi-stage stretching without a high-temperaturestretching step. As a result, a film having reduced thickness variation,less roughness of the surface and excellent abrasion resistance of thesurface can be obtained as well as the film forming speed can be greatlyincreased. Moreover, because high-temperature stretching is notnecessary, stretching rolls can be prevented from soiling, therebyimproving productivity of the film greatly.

It is evident that the longitudinally stretched film thus obtained maybe stretched in its transverse direction and/or heat treated as needed.Particularly, when the longitudinally stretched film according to thepresent invention is stretched in the transverse direction, a biaxiallystretched film having a low thickness variation, a high strength, a highYoung's modulus and excellent abrasion resistance and slipping propertycan be obtained as compared with a biaxially stretched film obtained bythe conventional multi-stage stretching process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an apparatus for producing abiaxially stretched film for carrying out a process for producing apolyester film according to an embodiment of the present invention.

FIG. 2 is an enlarged side view of a casting process of the apparatusshown in FIG. 1.

FIG. 3 is a perspective view of a water supply means shown in FIG. 2.

FIG. 4 is an enlarged sectional view of a water eliminating means shownin FIG. 2.

FIG. 5 is a perspective view of a water supply means different from oneshown in FIG. 3.

FIG. 6 is a schematic side view of a further different type water supplymeans.

FIG. 7 is a sectional view of a water eliminating means different fromone shown in FIG. 4.

FIG. 8 is a schematic side view of a further different type watereliminating means.

FIG. 9 is a schematic side view of a further different type watereliminating means.

FIG. 10 is a schematic side view of a longitudinal stretching processdifferent from one shown in FIG. 1.

THE BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be explained withreference to the drawings.

FIGS. 1-4 illustrate an apparatus for carrying out a process forproducing a polyester film according to an embodiment of the presentinvention. FIG. 1 shows the entire process from a die delivering amolten sheet to a film winder of an apparatus for producing a biaxiallystretched film.

In FIGS. 1 and 2, a molten polyester sheet 2 delivered from a die 1 as asheet configuration is cast onto the surface of a cooling drum 3constructed as a cooling medium in which cooling water is circulated,the cast sheet is sent in the circumferential direction of the drumtogether with the moving surface of the drum associated with therotation of the drum, and the sheet is solidified by cooling on thesurface of the drum. Cooled and solidified polyester sheet 4 leaves fromthe surface of cooling drum 3 at a position of a take-off roll 5 and issent to a next process.

Air containing water vapor is supplied from the inside of a cover 6which is constructed as a water supply means and opens towards thesurface of cooling drum 3 to the surface of the rotating drum, and awater layer 7 constructed from many and very small water drops is formedon the surface of the drum by bedewing the water vapor on the drumsurface. The average thickness of this water layer 7 is d.

Water layer 7 formed on the surface of cooling drum 3 moves togetherwith the surface of the drum in the circumferential direction of thedrum and reaches a meniscus portion 8 formed at the position wheremolten polyester sheet 2 comes into contact with the surface of thecooling drum. An amount of water conveyed by water layer 7 iscontinuously absorbed into meniscus portion 8, and the water interposedbetween molten polyester sheet 2 and the drum surface is continuouslyconveyed out from the meniscus portion as a water layer continuouslyextending in the transverse direction of the sheet. Since the amount ofthe water flown into meniscus portion 8 and the amount of the waterflown out from the meniscus portion are balanced, the meniscus portionis maintained substantially to a constant configuration. The freesurface of the water of meniscus portion 8 is formed as a curved surfaceby meniscus phenomenon, and the height h of the curved surface from thedrum surface is the height of the meniscus portion.

On the surface of cooling drum 3 after cooled and solidified polyestersheet 4 leaves from the drum, a non-uniform water layer 9 remains. Thiswater layer 9 is eliminated by an air suction roll 10 constructed as awater eliminating means. Water layer 7 with a required average thicknessis formed by bedewing water vapor supplied from the inside of cover 6 onthe drum surface after the remaining water is eliminated.

The water supply means is constructed in this embodiment as shown inFIG. 3. A pipe 12 is inserted into the hot water having a temperature ofnot higher than 100° C. and contained in a tank 11, air is supplied froman appropriate air supply means (not shown) into the hot water throughthe pipe and the hot water is bubbled. Much water vapor is included inthe air by this bubbling, and the air containing much water vapor isintroduced into cover 6 through a pipe 13. The air containing watervapor is supplied from the inside of cover 6 towards the surface ofcooling drum 3 having a temperature of 20°-30° C., and the water vaporis bedewed on the drum surface as fine water drops. The water drops formwater layer 7 having a required thickness.

Air suction roll 10 as a water eliminating means is constructed as shownin FIG. 4. Air suction roll 10 comprises a cylindrical hollow memberfree to rotate and having suction holes 14, a water absorbing material16 wrapped on the periphery of the member 15 (for example, a nonwovenfabric or a sponge) and a fixed shaft 18 having a fan-shaped guide 17opening towards the surface of cooling drum 3. A vacuum pump 20 isconnected to the end portion of fixed shaft 18 via a pipe 19, and theair inside of air suction roll 10 is sucked by the vacuum pump. Thewater absorbed by water absorbing material 16 is discharged togetherwith the above sucked air.

Polyester sheet 4 cooled and solidified on the surface of cooling drum 3is sent to a longitudinal stretching process.

Sheet 4 passes through a position between a pair of nip rolls 21 and 22,is heated to a predetermined temperature by preheating rolls 23, 24 and25 and enters into a longitudinal stretching process 26 as shown inFIG. 1. In this embodiment, the longitudinal stretching is conducted inthree zones 27, 28 and 29. The first stretching step is conductedbetween (a drive stretching roll 30 and a nip roll 31) and (a drivestretching roll 32 and a nip roll 33), the second stretching step isconducted between (the drive stretching roll 32 and the nip roll 33) and(a drive stretching roll 34 and a nip roll 35) and the third stretchingstep is conducted between (the drive stretching roll 34 and the nip roll35) and (a drive stretching roll 36 and a nip roll 37). The total drawratio in this three-stage longitudinal stretching is set to not lessthan 4.5 times the original length.

The longitudinally stretched film becomes an uniaxially stretched film38, the film 38 passes through a cooling rolls 39 and 40 and a nip roll41, and thereafter, the film is sent to a transverse stretchingapparatus 42.

The film stretched in the transverse direction by transverse stretchingapparatus 42 becomes a biaxially stretched film 43, and the film iswound as a spool by a winder 44.

In the above film producing apparatus, the water supply means in thecasting process may be constructed of a seepage roller 52 the surface ofwhich is covered with a water holding material 51 (for example, anonwoven fabric or a sponge) and the water supplied from the inside ofthe seepage roller may be applied onto the surface of a cooling drum 53through the water holding material, as shown in FIG. 5.

Further, a gravure coater system may be adopted as shown in FIG. 6. Inthe system shown in FIG. 6, the water stored in a tank 61 is conveyed bythe surface of a gravure roll 62, the conveyed water is transferred tothe surface of a transfer roller 63 and the water is applied onto thesurface of a cooling drum 64 from the surface of the transfer roller.

A water eliminating means other than one shown in FIG. 4 may be adopted.For example, as shown in FIG. 7, a suction box 72 having a slit 71 forsuction is provided, a water absorbing material 73 is attached to aportion in front of the slit, the water absorbing material is broughtinto contact with the surface of the cooling drum and air is sucked fromthe inside of the suction box. Further, air suction roll 10 shown inFIG. 4 and an air knife 81 may be combined as shown in FIG. 8. Althoughthe most of water drops 83 remaining on the surface of a cooling drum 82can be eliminated by air suction roll 10, a small amount of water drops84 remains on the surface of the cooling drum because water absorbingmaterial 16 covering the surface of the air suction roll is in a moiststate. This small amount of water drops 84 is blown off or dried by airfrom air knife 81 and substantially the water drops can be almostcompletely eliminated. Furthermore, the system shown in FIG. 9 may beadopted. Air from an air knife 91 is blown to a cooling drum 92, and thewater drops blown off together with the air and the water dropsremaining on the drum surface are sucked by another air knife 93. InFIG. 9, air knives 91 and 93 are constructed as an integral type doubleair knife and a water supply means 94 has a construction similar to oneshown in FIGS. 2 and 3.

As a longitudinal stretching process, a process shown in FIG. 10 may beadopted.

In the embodiment shown in FIG. 10, the longitudinal stretching isconducted by two steps consisting of a step in the first-stagestretching zone 101 where a low-speed stretching is conducted and a stepin the second-stage stretching zone 102 where a stretching up to atarget total draw ratio is conducted. The first-stage stretching isconducted between (a drive stretching roll 103 and a nip roll 104) and(a drive stretching roll 105 and a nip roll 106). Free rolls 107, 108and 109 are disposed between drive stretching rolls 103 and 105, andpolyester sheet 4 preheated is gradually stretched at a low stretchingspeed during passing through these free rolls. Rotational torque may beapplied to free rolls 107, 108 and 109 by torque motors etc. withoutcontrolling respective roll speeds, and thus, the mechanical loss ofeach rotational free roll can be extinguished by the applied torque. Thesecond-stage stretching is conducted between (drive stretching roll 105and nip roll 106) and (a drive stretching roll 110 and a nip roll 111).The total draw ratio in the longitudinal stretching shown in FIG. 10 isalso set to not less than 4.5 times the original length.

Next, methods for determining and estimating the characteristics in thepresent invention will be explained and examples and comparativeexamples determined and estimated in accordance with the methods will beexplained.

(1) Thickness Variation

The produced film is sampled at a length of 10 m in its longitudinaldirection and with the entire width in its transverse direction. Thethickness of this sample is continuously determined by a strain gauge,and a value calculated by dividing the thickness difference Δt betweenthe maximum thickness and the minimum thickness by the average thicknesst is defined as thickness variation and the thickness variation isindicated by percentage.

(2) Surface Roughness of Film

The surface roughness of a film is determined according toJIS-B-0601-1976 at a cut-off value of 0.25 mm. In this determination,the maximum roughness is Rt and the center line average height is Ra.

(3) Surface Haze

A value obtained by subtracting the internal haze determined in tetralinsolution from the total haze determined according to ASTM-D-1003(JIS-K6714) is defined as surface haze.

(4) Thickness of Water Layer

The average thickness d of a water layer is determined byinfrared-absorbing analysis method. In this determination, the surfaceof water is covered by a sheet and the infrared ray is transmittedthrough the sheet. More concretely, infrared absorption hygrometer"M-300" produced by Chino Kabushiki Kaisha (a Japanese company) is used,the calibration curve between the output of the meter and the thicknessof a water layer is obtained before the determination and the data aredetermined by the calibration curve. This calibration curve isdetermined as follows. First, water is bedewed on a test piece having asize of 10 cm square and constructed by plating a stainless steel platewith hard chrome (Rt=0.2 μm), a non-stretched polyester film having athickness of 50 μm is laminated on the bedewed water, water contents aredetermined at ten points from the upper side of this film by thehygrometer, and the average value of the output date is determined (a).The weight of this test piece is determined by a balance (b). The weightof the test piece and the film is determined after the bedewed water iseliminated (c). The weight of the water (e) is calculated by thefollowing equation.

    e=b-c

The specific gravity of water is defined as 1.0 and the thickness of thewater layer (d') is calculated from the area (100 cm²) of the test pieceand "e". Other data of "e" are determined by changing the condition ofbedewing. The relationship between (a) and (d') are obtained, and thisis defined as the calibration curve.

(5) Height of Meniscus Portion h

This is defined as a vertical height from the surface of a coolingmedium of the meniscus curved surface which is formed due to the surfacetension on a meniscus portion formed near a point where a molten sheetcomes into contact with the surface of the cooling medium. Since thedetermination of "h" requires high accuracy, the photograph of themeniscus portion was taken using a fiber scope etc., and thereafter, "h"was determined from the magnified photograph.

(6) Diameter of Water Drops and Number of Water Drops

After water drops are formed on the surface of a cooling medium, amicroscope is quickly set (about 30 seconds after), and after thephotograph is taken, the diameter and the number of the water drops inthe photograph are determined.

(7) Surface Roughness of Cooling Medium

This is determined based on JIS-B-0601-1976.

(8) Bad Formability due to Air Entrapment

A sheet on a cooling drum is observed. A case where entrapment of aircan be clearly observed is defined as bad formability and shown by amark of "x", and a case where it cannot observed is defined as goodstate and shown by a mark of "o".

(9) Irregularity due to Water Boiling

It is determined by visual observation whether the water on the surfaceof a cooling drum boils when a molten sheet comes into contact with thewater and the irregularity of the sheet surface due to the boilingoccurs or not.

A case where the irregularity occurs is defined as "bad" state and shownby a mark of "x", and a case where the irregularity does not occur isdefined as "good" state and shown by a mark of "o".

(10) Orange Peel-like Irregularity

This is a surface defect of a sheet caused by the transfer of the shapeof water drops to the surface of the sheet. A case where orangepeel-like irregularity occurs on the sheet after casting and the defectcan be clearly observed even in the film after biaxially stretching isdefined as "unusable" state and shown by a mark of "x". A case whereorange peel-like irregularity is observed in a cast sheet but it is notobserved in the film after biaxially stretching is defined as "usable"state and shown by a mark of "Δ". A case where orange peel-likeirregularity cannot be observed even in a cast sheet is defined as"good" state and shown by a mark of "o".

(11) Flatness

A cast sheet with its entire width is sampled by a length of 3 m and oneend of the sample is attached to a shaft having a flat surface. Afterthe sample is laid along the surface of a roll free to rotate which isspaced from the shaft by 2.5 m, the other end of the sample is loaded by50 g/mm² so that the load is applied uniformly over the entire width ofthe sample.

A string is stretched over the entire width of the sheet in thehorizontal direction at a central position of the sheet stretchedbetween the shaft and the rool, namely at a position far from the shaftor the roll by 1.25 m. The string is set so as to come into contact withthe sheet at least at one position. In this condition, if the sheet hasa bad flatness, the bad-flatness portions of the sheet are positionedspacedly from the string in the vertical direction. This distancebetween the string and the surface of the sheet is determined and thegrade is estimated by the following criteria. If the flatness of thesheet has no problem at all, the sheet comes into contact with thestring over the entire width.

o: The distance between the sheet and the string is less than 2 mm.

Δ: The distance between the sheet and the string is not less than 2 mmand less than 10 mm.

x: The distance between the sheet and the string is not less than 10 mm.

In a case where the distance between the sheet and the string is lessthan 2 mm, the sheet has no problem and the estimated result thereof isshown by a mark of "o". In a case where the distance is not less than 10mm, the sheet is not usable as a non-stretched sheet and the sheet isalso unusable as a sheet for stretching because of the outbreak ofwrinkles in stretching process, and therefore, the estimated resultthereof is shown by a mark of "x". In a case where the distance is notless than 2 mm and less than 10 mm, the flatness of the sheet isrecognized not to be good, but the sheet can be usable depending uponits usage, and therefore, the estimated result thereof is shown by amark of "Δ".

(12) Long Term Stability

This is estimated by the period of time in which the cracking of asheet, the slipping of the sheet or the snaking of the sheet occurs. Acase where there is no problem for a term of not less than eight hoursis defined as "good long term stability" state and shown by a mark of"o", and a case where there occurs any problem in a term of less thaneight hours is defined as "bad long term stability" state and shown by amark of "x".

(13) High-Speed Casting Property

A case where there is no defect in a cast sheet and no trouble in thecasting property and a casting speed of not less than 80 m/min. can beachieved is defined as "good high-speed casting property" state andshown by a mark of "o", and a case where only a casting speed of lessthan 80m/min. can be achieved is defined as "bad high-speed castingproperty" state and shown by a mark of "x".

(14) Curling Up of Sheet End Portions

This is estimated by the amount of the curling up of sheet end portionsduring casting. The state with no curling up is shown by a mark of "⊚",the state with a curling-up amount of not greater than 1 mm is shown bya mark of "o", the state with a curling-up amount of not less than 5 mmis shown by a mark of "x" and the intermediate state is shown by a markof "Δ". The measuring point is positioned upstream by 100 mm in thecircumferential direction of the cooling medium from a position wherethe sheet leaves from the cooling medium.

(15) Width of Water Application in Sheet End Portion

The position of a sheet edge is defined as the reference point, and thewidth of water application in a sheet end portion is shown by a distancefrom the reference point. The direction from the reference point towardsthe central portion of the sheet in the transverse direction of thesheet is indicated as negative direction (-) and the direction towardsthe outside portion in which the sheet does not exist is indicated aspositive direction (+).

EXAMPLES 1-3

Polyethylene terephthalate (ultimate viscosity in O-chlorophenol [η];0.65, TiO₂ having an average particle diameter of 300 m μ is added as anadditive by 0.1 wt.%) was dried under vacuum at a temperature of 180° C.by a regular method, the dried polyethylene terephthalate was suppliedto an extruder and molten therein at a temperature of 285° C., andthereafter, the molten polyethylene terephthalate was quantitativelymeasured by a gear pump and delivered out from a T-type die as a moltensheet having a constant thickness. The molten sheet was cooled andsolidified on a cooling drum having a mirror-finish surface (the surfaceroughness Rt: 0.1 μm) at a casting speed of 50 m/min., whileelectrostatic charge was applied to the sheet over the entire width ofthe sheet. In this casting, air containing saturated water vapor havinga temperature of 80° C. was blown to the drum surface controlled at atemperature of 25° C., and water drops were uniformly applied on thedrum surface so as to form a water layer having an average thickness dof 1 μm. The height h of meniscus portion formed between the moltensheet with electrostatic charge and the cooling drum was controlled to2.5 m.

The cast sheet thus obtained was longitudinally stretched at a drawratio of 1.8 times via stretching rolls heated at 100° C., andthereafter, stretched at respective draw ratios of 2.6 times, 3.6 timesand 4.9 times the original length via stretching rolls heated at 90° C.,and thus, the sheet was longitudinally stretched by two stretching stepsat a total draw ratio of 4.7 times, 6.5 times or 8.8 times the originallength, respectively. Successively, the longitudinally stretched filmwas stretched in the transverse direction in a tenter heated at atemperature of 95° C. and heat treated at a temperature of 210° C. for 7seconds relaxing the film therein by 5% in the transverse direction.Thus, a biaxially stretched polyester film having a thickness of 12 μmwas obtained.

In this film formation, stretching with a high draw ratio was possibleeven in the condition of a relatively low longitudinal stretchingtemperature, stretching rolls were not soiled by deposited material eventhough the stretching is conducted for a long term, and thus stablestretching was possible. Accordingly, a very high-speed stretching couldbe attained as well as the stable state of stretching could be obtained.The characteristics of the film thus obtained are shown in Table 1.

When the film having the above characteristics is used for a base filmof a magnetic tape, a magnetic tape having excellent running ability,slipping ability and abrasion resistance can be obtained.

COMPARATIVE EXAMPLE 1

The biaxially stretching and heat treatment were conducted after a usualelectrostatic casting method without a water layer was applied insteadof the casting method with a water layer used in Examples 1-3, and otherconditions were the same as in Example 1.

As a result, breakage of the film frequently occurred when the film wastransversely stretched, presumably because the film was orientated toomuch when its longitudinal stretching. Although a stable stretchingcould not be achieved, the data determined on the quality of thebiaxially stretched film are shown in Table 1.

COMPARATIVE EXAMPLE 2

The same casting method as in Comparative Example 1 was applied and thestretching temperature in the longitudinal stretching was controlled toa high temperature similar to the temperature in the conventionalmulti-stage stretching.

As a result, the stretching rolls were soiled in a short period of time,and the productivity was greatly decreased because of the cleaning ofthe stretching rolls. Moreover, the film adhered to the high-temperaturestretching rolls when the film was stretched and the film afterstretching had a rough surface.

COMPARATIVE EXAMPLE 3

The same casting condition as in Examples 1-3 was applied and thelongitudinal stretching was conducted by a usual single stagestretching. Although the casting speed was the same speed as in Examples1-3, the film forming speed after longitudinal stretching was suppressedto a low speed.

                                      TABLE 1                                     __________________________________________________________________________                                                Film Characteristics                          Longitudinal Stretching Conditions                                                                      Film  Thickness                                Casting                                                                            First Step Second Step                                                                              Total                                                                             Forming                                                                             Variation                                                                             Surface                                                                             Surface                    Water                                                                              Stretching                                                                           Draw                                                                              Stretching                                                                           Draw                                                                              Draw                                                                              Speed (longitudinal/                                                                        Roughness                                                                           Haze                       Layer                                                                              Temp. (°C.)                                                                   Ratio                                                                             Temp. (°C.)                                                                   Ratio                                                                             Ratio                                                                             (m/min.)                                                                            transverse)                                                                           Rt (μm)                                                                          (%)                 __________________________________________________________________________    Example 1                                                                            Formed                                                                             100    1.8 90     3.6 6.5 325   1.5/0.8 0.15  0.1                 Example 2                                                                            Formed                                                                             100    1.8 90     2.6 4.7 235   2.0/1.2 0.18  0.2                 Example 3                                                                            Formed                                                                             100    1.8 90     4.9 8.8 440   1.4/0.7 0.14  0.13                Comparative                                                                          Not  100    1.8 90     3.6 6.5 325   18/25   1.5   20                  Example 1                                                                            formed                                                                 Comparative                                                                          Not  120    1.8 110    3.6 6.5 325   19/21   1.7   18                  Example 2                                                                            formed                                                                 Comparative                                                                          Formed                                                                             100    3.5 --     --  3.5 175    9.5/10.3                                                                             1.3   13                  Example 3                                                                     __________________________________________________________________________

Next, the resulted data of the tests in accordance with the conditionsof the casting method utilizing a water layer will be explained.Firstly, the results with respect to the height h of a meniscus portionare shown in Example 4 and Comparative Example 4.

EXAMPLE 4

Polyethylene terephthalate (ultimate viscosity in O-chlorophenol[η];0.65, SiO₂ having an average particle diameter of 300 m μ is added as anadditive by 0.1 wt.%) was used as a polymer to be molten, thepolyethylene terephthalate was dried under vacuum at a temperature of180° C. by a regular method, the dried polyethylene terephthalate wassupplied to an extruder and molten therein at a temperature of 285° C.,and thereafter, the molten polyethylene terephthalate was quantitativelymeasured by a gear pump and delivered out from a T-type die as a moltensheet having a constant thickness. The molten sheet was cooled andsolidified on a cooling drum having a mirror-finish surface (the surfaceroughness Rt: 0.2 μm) at a casting speed of 100 m/min., whileelectrostatic charge was applied to the sheet over the entire width ofthe sheet. In this casting, air containing saturated water vapor havinga temperature of 80° C. was blown to the drum surface controlled at atemperature of 25° C., and water drops were uniformly applied on thedrum surface so as to form a water layer having an average thickness dof 1 μm. Thus, the molten sheet with electrostatic charge and having athickness of 100 μm was brought into contact with the drum on which thewater layer having an average thickness of 1 μm was formed, and theheight h of the meniscus portion formed at their contact position wascontrolled to 3 μm.

Although the casting was continued for a week under such a condition,there occurred no sheet-surface defects and no sheet end portiontroubles in the casting, and the sheet was cast in a stable state.

COMPARATIVE EXAMPLE 4

The height of the meniscus portion was altered by changing the positionapplied with electrostatic charge in Example 4 above the periphery ofthe cooling drum in the circumferential direction of the drum. Otherconditions were the same as in Example 4, and the sheet having aresulted data are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                               Comparative                                                          Example 4                                                                              Example 4                                              ______________________________________                                        Height of Meniscus Portion                                                                    3          0.9                                                h (μm)                                                                     Average Thickness of Water                                                                    1          1                                                  Layer d (μm)                                                               Obtained Sheet Surface                                                                        Good       Orange peel-like                                                              irregularity occurs                                ______________________________________                                    

Thus, it is understood that, when the height h of the meniscus portionis lower than the average thickness d of the water layer formed on thesurface of the cooling medium, the defect of orange peel-likeirregularity inevitably occurs on the surface of the cast sheet.

Next, the conditions for forming a water layer by bedewing method wereexamined.

EXAMPLES 5-7, COMPARATIVE EXAMPLES 5-7

The same polyethylene terephthalate as in Example 4 was used, it wasdried under vacuum at a temperature of 180° C., supplied to an extruderand molten at a temperature of 285° C., a molten sheet was delivered outfrom a T-type die, and electrostatic charge was applied to the sheetover the entire width of the sheet. Air with moisture shown in Table 3was sent onto a cooling drum (chrome-plated roll) shown in Table 3, awater layer formed by water drops shown in Table 3 was formed, and thesheet having a thickness of 100 μm was cast on the cooling drum at aspeed of 90 m/min.

The water eliminating roll shown in FIG. 2 was attached between theposition where the sheet left from the drum and the position where airwith moisture was blown, the roll was pressed onto the cooling drum witha pressure of 0.8 kg/cm, and surplus water was eliminated via a vacuumpump having a capacity of 500 l/m².min.

This cast sheet was sent to a regular biaxially stretching apparatus tostretch the sheet with one stretching step in the longitudinal directionand stretch the film with one stretching step in the transversedirection. The biaxially stretched film was thus obtained. The castingconditions was estimated from both of the cast sheet and the biaxiallystretched film. The resulted data are shown in Table 3.

As a result, a film without the forming defect due to the entrapment ofair, the irregularity of the film surface due to the boiling of waterand orange peel-like irregularity could be obtained by bedewing waterdrops having a specific configuration on a cooling drum having aspecific surface roughness, even if the casting was conducted at a highspeed.

                                      TABLE 3                                     __________________________________________________________________________           Conditions of                                                                 Cooling Drum                                                                            Conditions of                                                                          Shape of Water Drop                                             Surface                                                                            Moist Air                                                                              Maximum                                                                             Number    Defects of Film and Sheet                  Surface                                                                            Rough-   Amount                                                                             Diameter                                                                            of Water  Forming Defect                                                                        Irregularity                                                                         Orange                      Temp.                                                                              ness Temp.                                                                             of Air                                                                             of Drops                                                                            Drops     due to Air                                                                            due to                                                                               Peel-like                   (°C.)                                                                       Ra (μm)                                                                         (°C.)                                                                      (l/min.)                                                                           (μm)                                                                             (Number/0.1 mm.sup.2)                                                                   Entrapment                                                                            Boiling                                                                              Irregularity         __________________________________________________________________________    Comparative                                                                          25   0.047                                                                              80  50   95    52        ◯                                                                         ×                                                                              ×              Example 5                                                                     Example 5                                                                            25   0.038                                                                              80  50   69    55        ◯                                                                         ◯                                                                        Δ              Example 6                                                                            20   0.024                                                                              70  40   20    100       ◯                                                                         ◯                                                                        ◯        Example 7                                                                            15   0.010                                                                              70  30   10    480       ◯                                                                         ◯                                                                        ◯        Comparative                                                                          20   0.024                                                                              80  500  85    530       ◯                                                                         ×                                                                              ×              Example 6                                                                     Comparative                                                                          20   0.024                                                                              70   5    5    42        × ◯                                                                        ◯        Example 7                                                                     __________________________________________________________________________

Next, in the casting method for interposing a water layer, the effectaccording to enlarging the thickness of the water layer at the sheet endportions and the effect according to cooling the anti-cooling drum-sidesurface of the sheet at the sheet end portions were investigated.

EXAMPLES 8-11, COMPARATIVE EXAMPLES 8-11,

Polyethylene terephthalate (IV=0.65) was dried under vacuum at atemperature of 180° C., supplied to an extruder and molten at atemperature of 290° C., a molten sheet was delivered out from a T-typedie, and electrostatic charge was applied to the sheet over the entirewidth of the sheet. The sheet having a thickness of 50 μm wascontinuously cast onto a cooling drum having a surface roughness Rt of0.2 μm under the casting conditions of respective casting speeds of 30,70 and 100 m/min. for 24 hours, as shown in Table 4.

The water layer was formed by bedewing method and the thickness of thewater layer was controlled by the amount of sent moist air. The bedewingapparatus for the central portion of the sheet and that for the sheetend portions were different from each other. The water eliminating rollshown in FIG. 2 was attached, a pressure of 0.7 kg/cm was applied to theroll and surplus water was eliminated via a vacuum pump having acapacity of 500 1/m².min.

For cooling of the anti-cooling drum-side surface of the sheet at thesheet end portions, nozzles having a slit width of 0.2 mm and a lengthof 30 mm, in a state that the longitudinal direction of each nozzle wasset to the transverse direction of the sheet, were arranged by 15 innumber in the sheet running direction, and the amount of the watersupplied from the nozzles was controlled to prevent the water layerformed the supplied water from breaking.

As is evident from the result shown in Table 4, it is understood that acast sheet with an excellent flatness and less curling up of the sheetend portions can be obtained by specifying the distribution of thethickness of the water layer in the transverse direction of the sheet,and casting stable for a long term and high-speed casting can bepossible.

                                      TABLE 4                                     __________________________________________________________________________            Casting Conditions                                                            Conditions of Water Application                                               Thickness                                                                            Thickness                                                                            Width of                                                                              Cooling of                                                                             Characteristics                                of Water                                                                             of Water                                                                             Water   Anti-Drum                                                                              of Cast Sheet                                                                             Casting Properties                 Layer in                                                                             Layer in                                                                             Application                                                                           Surface of    Curling up   High-Speed                   Central                                                                              Sheet End                                                                            in Sheet End                                                                          Sheet End     of Sheet End                                                                         Long                                                                                Casting                      Portion (μm)                                                                      Portions (μm)                                                                     Portions (mm)                                                                         Portions Flatness                                                                           Portions                                                                             Stability                                                                           Property             __________________________________________________________________________    Comparative                                                                           0      0      --      Not conducted                                                                          ◯                                                                      ◯                                                                        ◯                                                                       ×              Example 8                                                                     Comparative                                                                           1.0    1.0    +10--100                                                                              Not conducted                                                                          ×                                                                            ×                                                                              ×                                                                             ◯        Example 9                                                                     Example 8                                                                             1.0    1.5    +10--100                                                                              Not conducted                                                                          ◯                                                                      ◯                                                                        ◯                                                                       ◯        Example 9                                                                             2.7    3.7    +10--100                                                                              Not conducted                                                                          ◯                                                                      ◯                                                                        ◯                                                                       ◯        Example 10                                                                            2.7    3.7    +10--100                                                                              Conducted                                                                              ◯                                                                      ⊚                                                                     ◯                                                                       ◯        Comparative                                                                           3.5    4.0    +10--100                                                                              Not conducted                                                                          ×                                                                            ◯                                                                        ×                                                                             ◯        Example 10                                                                    Comparative                                                                           0.05   0.7    +10--100                                                                              Not conducted                                                                          ×                                                                            ◯                                                                        ×                                                                             ×              Example 11                                                                    Example 11                                                                            0.2    4.5    +10--100                                                                              Not conducted                                                                          ◯                                                                      ◯                                                                        ◯                                                                       ◯        __________________________________________________________________________

INDUSTRIAL APPLICATIONS FOR THE INVENTION

As explained hereinabove, in the process for producing a polyester filmaccording to the present invention, since the film can be formed stablyand at a high speed and the film having good transparency, less surfaceroughness and excellent abrasion resistance can be obtained, the processis very useful for producing a polyester film for magnetic materials,electrically insulating materials, capacitors, other industrialmaterials and wrapping materials.

What is claimed is:
 1. A process for producing a polyester filmcomprising solidifying a molten polyester sheet by cooling it on thesurface of a mobile cooling medium on which a water layer is formed,said water layer formed on the surface of said mobile cooling medium ata position immediately ahead of the position where said molten polyestersheet comes into contact with the surface of said mobile cooling mediumand having an average thickness d, wherein a meniscus portion formed atthe position where said molten polyester sheet comes into contact withthe surface of said mobile cooling medium has a height h from thesurface of said mobile cooling medium such that h>d; the sheet is thenstretched at a total draw ratio of at least 4.5 times the originallength in a longitudinal direction through a plurality of stretchingsteps.
 2. The process according to claim 1, wherein said moltenpolyester sheet is solidified by cooling it on the surface of saidmobile cooling medium while electrostatic charge is applied to saidmolten polyester sheet.
 3. The process according to claim 1, whereinside portions of said water layer are formed thicker than the centralportion in the transverse direction of said molten polyester sheet. 4.The process according to claim 1, wherein the anti-cooling medium-sidesurface of said molten polyester sheet is cooled at its end portions inthe transverse direction of said sheet by a cooling means other thansaid mobile cooling medium.
 5. The process according to claim 1, whereinsaid water layer is formed by a method for bedewing water vapor on thesurface of said mobile cooling medium.
 6. The process according to claim5, wherein the maximum diameter of dew drops formed on the surface ofsaid mobile cooling medium by said bedewing method is not greater than70 μ.
 7. The process according to claim 5, wherein the density of dewdrops formed on the surface of said mobile cooling medium by saidbedewing method is not less than 50/0.1 mm² in number.
 8. The processaccording to claim 1, wherein water for forming said water layer issupplied onto the surface of said mobile cooling medium by a watersupply means after residual water on the surface of said mobile coolingmedium is eliminated by a water eliminating means at a position betweenthe position where the polyester sheet solidified by cooling on thesurface of said mobile cooling medium leaves from the surface of saidmobile cooling medium and the position where said molten polyester sheetcomes into contact with the surface of said mobile cooling medium. 9.The process according to claim 1, wherein the center line average heightRa of the surface roughness of said mobile cooling medium is not greaterthan 0.4 μm.
 10. The process according to claim 1, wherein the drawratio of each stretching step of said plurality of stretching steps inthe longitudinal direction is at least 1.1 times.
 11. The processaccording to claim 1, wherein, in said plurality of stretching steps inthe longitudinal direction, the draw ratio of a stretching step at adownstream position in a sheet running direction is greater than that ofa stretching step at an upstream position in the sheet runningdirection.
 12. The process according to claim 1, wherein each stretchingstep of said plurality of stretching steps in the longitudinal directionis conducted between an upstream drive stretching roll and a downstreamdrive stretching roll which are disposed adjacent to each other in asheet running direction.
 13. The process according to claim 1, wherein,in said plurality of stretching steps in the longitudinal direction, afinal stretching step is conducted between an upstream drive stretchingroll and a downstream drive stretching roll which are disposed adjacentto each other in a sheet running direction, and stretching stepsupstream of said final stretching step in the sheet running directionare conducted between an upstream drive stretching roll and a downstreamdrive stretching roll, which are disposed in the sheet runningdirection, via free stretching rolls which are disposed between thedrive stretching rolls.